The invention relates to a method for producing surfactant and microemulsion templated thin films and more particularly, to an evaporation-induced method for producing high-porosity, surfactant and microemulsion templated hybrid (inorganic/organic) and composite thin films.
Hybrid organic/inorganic films with controlled pore structure and surface chemistry are of interest for a range of applications including membranes, sensors, low dielectric constant (low k) films, photonic materials, and optical hosts. Films with controlled pore structure and high porosity (that is, greater than 50%) are particularly attractive to applications requiring materials with low dielectric constants (for example, dielectric constants less than 2). Surfactant templating is a rather recent approach toward achieving pore size control of inorganic frameworks, and so-called hybrid sol-gel chemistry provides a convenient route to derivatize the pore surfaces with covalently-bonded organic ligands. Burkett et al. (Burkett, S., Sims, S., and Mann, S., Chemical Communications, 1996, 11, 1367-1368), Fowler et al. (Fowler, C., Burkett, S., and Mann, S., Chemical Comm., 1997, 1769-1770), and Lim et al. (Lim, M., Blanford, C., and Stein, A., J. Amer. Chem. Soc., 1997, 119, 4090-4091) recently combined these approaches to form hybrid inorganic/organic mesoporous silica. Their synthesis procedures involved reacting tetraalkoxysilanes (Si(OR).sub.4, where R=ethyl or methyl) and an organoalkoxysilane (R'Si(OR).sub.3, where R' is a non-hydrolyzable organic ligand) with water under basic pH conditions in the presence of surfactant (cetyltrimethylammonium bromide) with initial surfactant concentration c.sub.o greater than the critical micelle concentration (cmc). These procedures result in the precipitation of powder. Various acid/solvent extraction procedures were used to remove the surfactant, resulting in organically-modified mesoporous powders with 1-dimensional, hexagonal architectures. The organic ligands in the mesoporous products included vinyl, phenyl, n-octyl, 3-sulfanylpropyl, aminopropyl, 2,3-epoxypropoxy, and imidazole. In these examples, the hybrid mesoporous silica was a powder, precluding its use in such promising applications as membranes, low k films, and optically-based sensors that generally require transparent, defect-free, supported thin films.
A second general approach to preparing hybrid-mesophases is to prepare a stable, mesoporous silica product and then to react the pore surfaces with various organic groups using standard silane coupling chemistry. For example, Feng et al. (Feng, X., Fryxell, G. Wang, L. Kim. A., Liu, J. and Kemner, K., Science, 1997, 276, 923-926) prepared mesoporous silica products using cetyltrimethylammonium chloride as the surfactant template. After calcination, the mesoporous silica was reacted with trimethoxymercaptopropylsilane. The powder was used to remove mercury and other heavy metals from contaminated solutions. Mesoporous silicas have also been organically-derivatized via vapor phase techniques. These powders suffer the same limitations as described above.
Sellinger et al. (Sellinger, A., Weiss, P., Nguyen, A., Lu, Y., Assink, R., Gong, W., and Brinker, C., Nature, 1998, 394, 256-260; incorporated herein by reference) describe a solvent evaporation technique to form ordered structures through a liquid phase process, but with little or no porosity. Brinker et al. (U.S. Pat. No. 5, 858,457, issued on Jan. 12, 1999) describe a solvent-evaporation method to form mesostructured films using metal oxides but the described process does not provide for the preparation of hybrid inorganic/organic and composite thin films. Brinker et al. also do not provide for covalently bonding ligands to the porous film structure or for entrapping molecules within the pores. Useful would be a liquid-phase method to form highly porous thin films using a solvent evaporation technique with essentially uni-modal pore size distributions and high surface areas.